1. Field of the Invention
This invention relates to image transmitting optical systems and, more particularly, to image transmitting optical systems for rigid endoscopes and the like in which an image of an object is transmitted through a plurality of polymeric relay lenses.
2. Description of the Related Art
Endoscopic image transmitting optical systems are extensively used to permit visualization of typically inaccessible areas within a patient's body. These systems generally include a housing enclosing a lens portion for focusing and relaying an image, and an illumination means for illuminating the region to be viewed. The lens portion used for focusing and relaying the images has typically been formed using a plurality of precision made glass lenses making up an objective portion, a relay portion and an ocular portion.
Conventional glass image transmitting optical systems have made considerable use of rod lenses and glass plano cylinders in their relay portions. See, for example, U.S. Pat. Nos. 3,257,902 (Hopkins), 3,297,022 (Wallace), 4,036,218 (Yamashita et al.) and 4,148,550 (MacAnally). These lenses are typically arranged end to end with the refractive index of the rod lenses being greater than the medium separating them and the axial length of each of the rod lenses being greater than the axial length of the medium. By using rod lenses, the major part of the length of the optical path through the optical system is provided by the lens material and the divergence of the rays in the lenses is reduced.
One major drawback to glass lens assemblies, both thin lens and rod lens types, are their relatively high costs to manufacture and assemble. Glass lenses must be carefully and precisely ground to provide acceptable transfer optics. Also, the lens assemblies must be put together into an image transmitting optical system with great precision to insure proper spacing between the lenses. This results in an expensive and relatively delicate instrument which must be handled with great care.
A second drawback to endoscopes incorporating glass lens assemblies is that due to their relative expense, they must be reused with a number of patients. Given the risks of disease transmission, this necessitates that the endoscopes undergo severe sterilization procedures. The method currently employed to effectively sterilize the instrument is gas sterilization using ethylene oxide gas. This method is extremely slow (up to 24 hours) and must be carefully controlled. Also, the ethylene oxide gas may have a deleterious effect on any plastic parts of the endoscope which, in turn, may limit the effective life span of the instrument.
Because of the aforementioned drawbacks, effort has been focused on providing an image transmitting optical system for endoscopic use which is sufficiently simple and inexpensive to fabricate and assemble in quantity such that it can be disposed of after use. For example, U.S. Pat. No. 4,784,118 to Fantone et al. suggests an optical viewing device which uses a relay lens portion composed of a plurality of injection molded rod lenses arranged in end to end fashion. Although the use of injection molding to fabricate the rod lenses does reduce the cost of grinding the lenses from optical glass, it still requires precision molding since, in order to obtain a bright image, the polymeric lenses must be much longer than their diameter. The fabrication of such polymeric rod lenses is difficult and expensive using conventional manufacturing techniques. To overcome this problem U.S. Pat. No. 4,964,710 to Leiner discloses a hybrid glass and polymeric lens optical transmitting system wherein glass plano cylinders are disposed between molded polymeric curved surface lenses. While providing clinically acceptable results, the inclusion of glass plano cylinders continues to contribute to increased expense as well as increased weight to the assembled optical system.
Accordingly, there is a clear need in the art to provide a rigid image transmitting optical system composed exclusively of polymeric thin lenses which would provide clinically acceptable optical quality, particularly in endoscopic and/or laparoscopic procedures.